1. Field of the Invention
This invention relates to a device for measuring wrenching height of polygonal head fasteners, particularly hexagonal head fasteners.
2. Description of the Prior Art
Hexagonal head screws, bolts or nuts must all meet minimum tolerances in order to avoid damage and to ensure proper functioning of the fastener during tightening and loosening operations. One of the characteristics of these fasteners is what is referred to as wrenching height. Wrenching height is the distance from the hex portion of the head where it first exceeds a minimum across corners dimension to the very bottom of the head. This characteristic indicates whether or not the hex is "filled out" sufficiently to ensure that a driver or socket engages the head in a manner so that the fastener can be driven without rounding off of the corners. If the wrenching height is outside of tolerances, torque applied to the head will likely damage the fastener during tightening and loosening.
Prior art methods of gaging wrenching height are difficult to carry out and are extremely time consuming. For these reasons, the wrenching height is not commonly gaged by fastener suppliers or end users. Prior art methods of measuring wrenching height require two separate readings or measurements. These methods typically involve providing a gaging ring having an inner diameter which is equal to the minimum across corners width of the hex head being measured. By stacking one, two, or three gaging rings over the top of the hex head, at right angles to the axis of the screw or bolt, acceptable wrenching height can be determined. At least 60% of the head height must protrude beyond the lower surface of the bottom ring to meet most minimum acceptable tolerances.
During measuring a hex head screw or bolt is placed in an indexing plate wherein a washer portion of the hex head seats against an upper surface of the indexing plate. A measurement is then taken after placing a gaging ring over the hex head. The hex head bolt is then removed and the gaging ring is lowered against the upper surface of the indexing plate so that a second measurement can be taken. The difference between the two readings is equal to the protrusion of the head beyond the gaging ring. If a washer is seated against the indexing plate, the washer thickness must be accounted for by subtracting this from the originally measured value of the hex head.
One of the problems associated with the prior art methods of measuring wrenching height is that micrometers used for this purpose are not commercially available. Each supplier or user will thus have a micrometer specially made for measuring wrenching height so that the micrometers will vary from user to user. During measuring, the gaging rings must be handled frequently and because the gaging rings are relatively small, they are easily misplaced or lost. Replacement of the gaging rings is usually quite expensive as they are manufactured with very close tolerances.
What is needed is a hex wrenching height gage which is easy to use and can determine the gaging height without repeated measurements and calculations, and which also minimizes the loss or mishandling of gaging rings used during measurement.